The present invention relates to apparatus for detecting the presence of an object within a surveillance zone and more particularly to apparatus employing a varying magnetic field for detecting a body of high permeability material.
In French Pat. No. 763,681 granted to Mr. Pierre Arthur Picard on Feb. 19, 1934 there is described apparatus for locating objects by modifying a magnetic field. The different characteristics of conductive, resistive, low permeability and high permeability material are discussed, it being observed that in a varying magnetic field magnetized iron will produce perturbations containing even harmonics of the field frequency, while other permeable material will produce signals containing odd harmonics with greater permeability giving rise to higher order odd harmonics.
Detection of such material is accomplished generally by measuring the amplitude and phase of an odd harmonic relative to that of the field producing fundamental frequency signal. In the described transmitter, undesirable harmonics are eliminated first by tuning the transmitting coil in a series resonant circuit. In the receiver, a filter eliminates the fundamental frequency and any undesired harmonics and passes selectively the desired harmonics. For example, a piece of permalloy can be detected by passing the eleventh harmonic and all higher harmonics. On the other hand, a magnetized metal piece can be detected by measuring the even harmonics. It is also observed that detection may be effected by determining the quotient of the values of the fundamental frequency and its harmonic or harmonics, or the quotient of the harmonics alone.
Various coil structures are described in said French patent for transmitting and receiving the electromagnetic signals. In one embodiment a figure "8" coil frame is used for transmission while a rectangular coil frame is used for reception, and in another embodiment the coil frames are interchanged with the rectangular frame being used for transmission and the figure "8" frame being used for reception.
Subsequent to the granting of the Picard French patent, others have attempted to improve upon the so-called magnetic detection system. For example, in Bakeman, Jr. et al. U.S. Pat. No. 3,983,552, issued Sept. 28, 1976, there is disclosed a pilferage deterrent marker of laminated construction containing an easily magnetized layer of Permalloy and a control layer of difficult to magnetize Vicalloy or Remendur. Such marker, when the control layer is magnetized, is detected by a circuit responding to the amplitude and phase of the received second harmonic signal. That is, the phase of the incoming signal is compared with the phase of a local reference signal and if it is either in phase or 180.degree. out of phase and exceeds a given amplitude, an alarm will be triggered. Said Bakeman, Jr. et al. patent observes that when their control element is demagnetized there is practically no contribution from the even harmonics. What is present, apparently, is undetectable and is speculated as possibly due to the fact that a small bias may still remain due to the magnetic field of the earth or other magnetized objects.
In Purinton et al. U.S. Pat. No. 4,063,230, issued Dec. 13, 1977, there is described a system that monitors both the amplitude and the phase of the incoming signal and that triggers an alarm when both quantities fall within a predetermined range. The patent does not disclose the frequencies or harmonics that are employed. The antennas or coils are located in "facades" disposed in parallel relationship on opposite sides of a passageway to be controlled.
The intent of all the prior workers has been to improve the reliability of detection of the special high permeability tags or markers while avoiding false alarms associated with other objects having similar but not identical conductive and magnetic properties. Unfortunately, certain of the techniques employed give rise to other problems encountered in pilferage control. A viable system must reliably respond to the marker when the marker is within the surveillance zone but must not be triggered by markers outside of the zone, and it must be possible to confine the zone to a reasonable area. Confining the area covered can be accomplished by minimizing the transmitted power and selecting appropriate directional coil geometry. However, this is not a simple problem to solve because it is also necessary that the system be effective to detect the presence of a marker regardless of its orientation within the surveillance zone relative to the transmitting and receiving coils.
With the foregoing in mind, it is an object of the present invention to provide apparatus employing a varying magnetic field for detecting a marker which apparatus is adapted to couple effectively with markers within a surveillance zone substantially independent of the orientation of the latter and which functions with comparatively low power, considering the frequencies involved, so as to confine the interrogating field substantially to the surveillance zone.